Homework answers / question archive / ENERGY STORAGE SYSTEMS System Description In this homework, you are asked to conduct a thermodynamic analysis on compressed air energy storage (CAES) systems

ENERGY STORAGE SYSTEMS System Description In this homework, you are asked to conduct a thermodynamic analysis on compressed air energy storage (CAES) systems

Electrical Engineering

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ENERGY STORAGE SYSTEMS

System Description

In this homework, you are asked to conduct a thermodynamic analysis on compressed air energy storage (CAES) systems.

The proposed integrated system is shown in Figure 1. The power plant comprises two main subsystems including the solar-driven and compressed air energy storage (CAES) systems.

Firstly, the air is compressed by consuming the available electricity in the grid to activate a motor-compressor train. The thermal energy produced during two-stage compression is removed by intercoolers and after-coolers. Subsequently, the high-pressure and high

temperature air leaves the after-cooler at state (5). The solar-driven system including several

huge parabolic trough collectors (PTC) are supposed to transfer the solar energy to the heat

transfer fluid.

The power plant works in 3 different scenarios: a) Charging period, b) Storing period and c)

Discharging period. During the charging period (generally during off-peak times), high

temperature and high-pressure air leaving the after-cooler (state 5) is divided into two streams

(states 6 and 17). Specific fraction of the air enters into the expansion train with recuperator.

The rest part of the hot and pressurized air flows through the packed bed thermal energy storage

system to transfer its thermal energy to the rock bed as storage energy. This system is addedto

the integrated system in order to minimize heat losses in the underground cavern. Then, high

pressure and low-temperature air passes into the underground cavern.

During the storing period, flow in (17) is shut-downed by the control valve, so, flow at

(5) is not divided into two streams as in the charging period and the whole part of the

compressed air enters to the expansion train with recuperator. On the other hand, during the

discharging period (at peak times), flow in the (19) numbered pipeline is released by a control

valve and pressurized air starts to flow into the packed bed thermal energy storage system to

receive the heat from the packed rock and leaves the storage system at higher temperatures.2

Later, heated air is connected to the (6) numbered pipeline and through increasing the air mass

flow rate, excess electricity can be obtained from the integrated system.

Figure 1- Scheme of an integrated energy system with compressed air energy storage

equipments.

Given Data and Assumptions of the Designed System

Thermodynamic properties of the all streams of the charging period are listed in Table 1.

Table 1 – Thermodynamic properties of charging period.

State Fluid Type ??? ? (kg/s) Pi (kPa) Ti (K)

1 Air 10 101.3 298

2 Air 10 1250 631.6

3 Air 10 1250 398.1

4 Air 10 14420 819.4

5 Air 10 14420 550

6 Air 7 14420 550

7 Air 7 14420 5503

8 Air 7 14420 650

9 Air 22 12807 1117

10 Air 22 3000 899

11 Air 37 6600 1079

12 Air 37 1000 700

13 Air 37 1000 630

14 Air 29.34 101.3 298

15 Air 29.34 101.3 380

16 Air 29.34 101.3 470

17 Air 3 14420 550

18 Air 3 14420 353

19 Air

No flow during the charging period

20 Air

21 Air 30 12000 1350

22 Air 15 12000 1350

23 Air 15 12000 1350

Thermodynamic properties of the all streams of the discharging period are listed in Table 2.

Table 2 – Thermodynamic properties of the discharging period.

State Fluid Type ??? ? (kg/s) Pi (kPa) Ti (K)

1 Air 10 101.3 298

2 Air 10 1250 631.6

3 Air 10 1250 398.1

4 Air 10 14420 819.4

5 Air 10 14420 550

6 Air 10 14420 550

7 Air 22 14420 438

8 Air 22 14420 550

9 Air 37 13479 861.1

10 Air 37 2500 639.1

11 Air 52 5160 838.2

12 Air 52 150 580

13 Air 50 150 530

14 Air 29.34 101.3 298

15 Air 29.34 101.3 380

16 Air 29.34 101.3 470

17 Air No flow during the discharging

18 Air period

19 Air 12 14420 283

20 Air 12 14420 363

21 Air 30 12000 1350

22 Air 15 12000 1350

23 Air 15 12000 1350

Main assumptions of the overall system are given in Table 3.

Table 3 – Main assumptions of the designed system.

Parameter Value

Ambient temperature, T0 298 K

Ambient pressure, P0 101.325 kPa

Temperature of the sun, Tsun 6000 K

Charging period, tcharging 8 h

Discharging period, tdischarging 4 h4

Solar heat input during the charging and discharging periods 152 MW

Required electrical power of fan of the solar driven system during the

charging and discharging periods 3 MW

Question 1#

Write mass, energy, entropy and exergy balance equations for each of the system components.

Question 2#

Determine the required electrical work for the low and high-pressure compressors,

respectively.

Question 3#

Calculate the produced electrical work through high and low pressure turbines, respectively.

(For both charging and discharging periods)

Question 4#

Determine the both overall energy and exergy efficiencies of the system in the charging and

discharging periods, respectively.

Question 5#

Calculate the round trip efficiency of the packed bed thermal storage system based on the

given numerical data.

Question 6#

Determine the amount of heat losses of the underground cavern based on the given numerical

data assuming that it takes place only during the storing period.

Question 7#

Select any two effective variable in the combined cycle and conduct parametric study to

investigate their effect on the energy and exergy efficiency of the system.

Remarks:

• All assumptions and simplifications should be made in a conceptually correct manner

and listed accordingly.

• EES software or a similar one can be used in the calculations.

• Solar heat input during the charging and discharging periods= (152 W)